A battery having a large charge/discharge capacity density such as a lithium ion battery is not frequently used alone. The battery is used in a state of being installed in a battery-operated device, as a battery pack housed in a synthetic resin-made housing together with internal components that include a charge/discharge protection circuit for protecting the battery by interrupting conduction when an excessive current flows as in the case of short-circuiting of a battery terminal or when the battery is overcharged or overdischarged.
Such a battery pack is used as a power source for various electronic devices such as computers, mobile phones, PDA's, electric tools, and transportation equipment, and so it is inevitable that water is spilled over the battery pack or the battery pack is dropped into water in everyday life. Since water adhering to a connector of the battery pack in this case has a sufficient electric conductivity for allowing a current to flow through water, there is a problem that an abnormal condition (for example, melt of a connector part or its vicinity) is caused by heat generation due to short-circuiting between positive and negative electrodes of the connector or an electric circuit failure may occur.
For example, Japanese Patent Application Laid-Open No. 2003-132862 describes a battery pack having a battery and a battery protection circuit substrate. The battery pack includes: a leak detection circuit that outputs a leak detection signal when a current leak detection electrode (water sensor) conducts; a charge semiconductor switch control circuit that interrupts a charge semiconductor switch according to any of the leak detection signal and an overdischarge detection signal output upon overcharge detection of a battery protection circuit; and a discharge semiconductor switch control circuit that interrupts a discharge semiconductor switch according to any of the leak detection signal and an overdischarge detection current output upon overdischarge detection of the battery protection circuit. The leak detection electrode is connected to an electrode of one polarity of the battery, and the leak current detection circuit, the charge semiconductor switch control circuit, and the overdischarge semiconductor switch control circuit are connected to an electrode of an opposite polarity of the battery.
Though describing a structure of detecting water in a battery pack by a current leak detection electrode (water sensor), Japanese Patent Application Laid-Open No. 2003-132862 does not give any consideration to an abnormal condition that is caused by heat generation due to water adhesion to a connector part.